In interferometry, a reference signal is compared to a measured signal. The difference between the frequencies of these two signals is used to determine the velocity of an object. The phase difference between these two signals is used to determine the position of the object. The invention relates to an apparatus used to precisely measure cumulative phase differences between high frequency signals. The invention would also be useful in areas outside of interferometry, wherever precision measurements of cumulative phase differences between high frequency signals is desired.
The prior art requires that an apparatus which quickly measures small phase differences between signals must use a high-speed clock with a frequency on the order of the signal frequency divided by the fraction of a wavelength over which the phase difference is to be discernable. For example, if the measuring device has a clock frequency that is twice the signal frequency, then prior art measuring apparatus could measure a half wavelength phase difference. Whereas if the clock frequency is four times the signal frequency, a prior art phase difference measuring apparatus could measure a one-fourth phase difference.
There are devices in the prior art which do not require high clock speeds, but these devices are slow when measuring large cumulative phase differences with a high precision.